Ellipsometry is a powerful technique for evaluating thin films on semiconductors. In any ellipsometer, a probe beam having a known polarization state is directed to interact with a sample. Changes in the polarization state induced by the interaction of the probe beam with the sample are then monitored. This information, in the form of Ψ and Δ measurements, is used to analyze characteristics of the sample.
It is well known that in order to improve an ellipsometric analysis, some form of multiple measurements is desirable. This can include taking measurements at multiple angles of incidence or at multiple wavelengths. Spectroscopic ellipsometers using monochrometers to scan the wavelength have been known for many years. For example, see “High Precision Scanning Ellipsometer,” by Aspnes and Studna, Applied Optics, Vol. 14, No. 1, January 1975.
More recently, efforts have been made to expand the desired wavelength range of spectroscopic ellipsometers into the UV and to obtain measurement data at multiple wavelengths simultaneously. When used in the semiconductor field, broadband probe beams must be focused to a relatively small spot size on the sample surface. Attempts to use lenses (refractive optics) to focus broadband probe beams, and particularly those including UV wavelengths, have run into problems. First, lenses typically have chromatic aberrations. Such aberrations cause the focused distance to be different for different wavelengths. Another problem with lenses is that it was often difficult to find lens materials with good transmission characteristics across a broad wavelength range.
Due to these difficulties, researchers in the prior art began using curved mirrors to focus the broadband probe beam onto the sample surface. Mirrors are advantageous since they can be highly reflective across a broad range of wavelengths. In addition, mirrors exhibit little or no chromatic aberrations. The use of focusing mirrors for a broadband ellipsometer are described in U.S. Pat. No. 4,790,659, issued December 1988, to Erhman, and U.S. Pat. No. 5,608,526, issued Mar. 4, 1997, to Piwonka-Corle.
Unfortunately, while providing a solution for chromatic aberration, mirrors are relatively difficult to align. More specifically, since mirrors must be focused off-axis, any angular error in alignment creates twice that error in beam position. Further, if the mirror is removed from the optics path, a light beam cannot be used to align the rest of the optics path since the mirror is needed to turn the beam. In contrast, the elements of an optics system can be aligned when a focusing lens is removed from the beam path. Another problem with mirrors is that when the probe beam reflects off the mirror, the polarization state of the beam is changed. Such changes must be very accurately and precisely characterized, otherwise they will cause errors in the analysis of the sample.
Accordingly, it would be desirable to have an all-refractive (lens-based) system for focusing a broadband probe beam to a small spot onto a sample surface. Such a system would be easier to align. In addition, any polarization changes in the probe beam induced by the lens system can be more accurately controlled.